The present invention relates to a process for producing alkadienols. More particularly, it relates to a process for producing alkadienols which are dimerized hydrates of conjugated alkadienes, comprising reacting conjugated alkadienes with water in the presence of carbon dioxide using a palladium compound and a phosphine compound as catalyst.
Alkadienols, especially octadienols such as octa-2,7-diene-1-ol are important compounds in chemical industries as an intermediate for the production of n-octanol, its esters and the like. As the process for producing such alkadienols, a process comprising reacting conjugated alkadienes and water in the presence of carbon dioxide using a palladium compound and a phosphine compound as catalyst is known from, for instance, Chemical Communications. 330 (1971), and Japanese Patent Publication (KOKOKU) No. 50-10565. It is also known that triphenylphosphine is preferred as the phosphine compound used as ligand of the palladium compound, but even in this case, the yield of alkadienols and the selectivity for the desired octa-2,7-diene-1-ol are still unsatisfactory. It is further known from the above-mentioned Chemical Communications, that when triphenylphosphine is used in an amount of more than about 6 times by mole of palladium, the yield of alkadienols is decreased. Thus, there is a problem that this method was subject to restrictions on its operating conditions.
In Japanese Patent Application Laid-Open (KOKAI) No. 64-85988, there is described that as catalyst of the reaction, a low-valency palladium complex containing a ligand such as tri-substituted phosphine or a chemical species prepared by reducing a palladium (II) compound in the presence of a ligand such as trisubstituted phosphine is used. As one of the problems of the telomerization reaction using a palladium catalyst, there is described that in case the telomerization reaction is carried out by using a trisubstituted phosphine in excess of palladium, a long induction period is required for the reaction even when a low-valency palladium complex which is assumed as a catalytic activation species and prepared by using a palladium compound and a trisubstituted phosphine, is used as catalyst for the telomerization reaction. According to Japanese Patent Application Laid-Open (KOKAI) No. 64-85988, it is disclosed that the above reaction proceeds without a long induction period when the reaction is carried out in the presence of a telomerization catalyst composed of a palladium compound and a phosphonium salt which has been transformed into a specified structure as a phosphorous compound. Thus, the Japanese Patent Application Laid-Open (KOKAI) No. 64-85988 suggests that even if a phosphine compound is added to the reaction system as a catalyst component, it is transformed in the reaction system and exists as a phosphonium salt during and at the completion of the reaction.
It has been known to use a palladium compound and a phosphine compound as catalyst of the above-mentioned reaction, but full investigations have not been made on the form of the catalyst in the reaction solution.
In a complex catalytic reaction, the metallic component of the catalyst plays an important role, but the selection of the kind of the ligand used and the reaction conditions is also closely connected with the catalytic reaction activity and selectivity, and even when using a catalyst which has been popularly used in the art, the desired reaction result may not be obtained depending on the form of the catalyst in the reaction solution.
The present inventors have made extensive researches for providing an industrially advantageous process for the preparation of alkadienols, which is capable of producing a desired alkadienol in a high yield and with high selectivity from a dimerization hydration reaction which comprises reacting a conjugated alkadiene and water in the presence of carbon dioxide by using a palladium compound and a phosphine compound as catalyst, and as a result, have found that quite surprisingly a desired alkadienol, specifically octa-2,7-diene-1-ol in case of using 1,3-butadiene as conjugated alkadiene, can be obtained in a high yield and with high selectivity even at a low palladium concentration when a phosphine compound used as a catalyst component is allowed to exist in the reaction system not in the form of a phosphonium salt or phosphine oxide in the conventional reaction solution but in the form of a free phosphine compound. The present invention has been attained on the basis of this finding.